The present invention relates to an improved fixed displacement pump arrangement for satisfying the requirements of a hydraulic system while requiring less power to operate during low demand conditions, and more particularly, to a pump construction which prevents fluid flow when the full volume of fluid displacement from the pump is not required by the hydraulic system.
It is common in earth-moving equipment, such a front end loaders, backhoes or the like, to have a bucket or a shovel mounted on a tractor to be raised and lowered, tilted, or otherwise moved into the correct attitude by an appropriate mechanism for the work being performed at the moment. Such adjustments of the bucket or shovel are commonly made by hydraulic cylinders supplied with fluid pressure from a suitable pump.
A common mode of operation in earth-working is to move a bucket or shovel into a a pile of material. The hydraulic systems for such earth-working applications require a high volume of fluid at low pressure to rapidly move the cylinder piston rods and, therefore, the bucket or shovel to the work. Then, low fluid volume under high pressure must be available to provide the necessary tilting of the bucket or shovel to break a portion of the material loose from the work pile or lift the material in the bucket or shovel.
One of the prior art approaches has been to provide a fixed displacement pump to supply the required fluid under pressure with the excess being discharged through a relief valve. It is a common arrangement to use the tractor engine for driving the pump, and the pump is normally continuously delivering its maximum amount of fluid because the tractor engine runs at a governed speed. Much of the time, the full volume of fluid is not required, and the excess fluid power must be absorbed by the system in the form of undesired heating and wear on the relief valve.
Another prior art approach has been to utilize a variable displacement pump in connection with automatic controls so that the output of the pump can be maintained at a minimum except when further output is demanded by the system. A system so equipped demands less power to operate the hydraulic system, reduces the heat rise in the fluid when operating in a low demand condition, permits a possible reduction in capacity of an oil cooler, and reduces pump and relief valve noise under low demand conditions.
A major disadvantage to the use of a variable displacement pump is cost. A variable displacement pump is significantly more expensive than a comparable fixed displacement pump, increasing the overall expense of manufacturing earth-working machines such as backhoes and front end loaders.
Thus, there has been a need for an improved fixed displacement pump arrangement which is capable of reducing excess fluid volume during low demand operation, thereby requiring less power to operate while being less expensive than a comparable variable displacement pump.